The present invention relates to a low-foaming water-based printing ink composition compounded with a unique silicone-based defoaming composition. More particularly, the invention relates to a novel water-based printing ink composition compounded with a silicone-based defoaming composition capable of exhibiting particularly excellent defoaming activity when compounded in a water-based printing ink composition.
Needless to say, the productivity of printing works by the use of a water-based printing ink composition largely depends on the performance or printability behavior of the printing ink composition throughout the printing process including the steps of printing on a printing press, drying and post-treatment for finishing. Water-based printing ink compositions in general in the course of the printing work, for example, are always under a high shearing force by rollers rotating at a high velocity so that foaming of the water-based printing ink composition is more or less unavoidable and the productivity of the printing work is sometimes greatly decreased due to the troubles such as eventual overflowing of the foamed ink composition from the ink feeder, irrelegularity or unevenness of printing density and so on.
Accordingly, it is a usual practice, with an object to decrease foaming of a water-based printing ink composition, to admix the same with a defoaming composition or antifoam agent, of which silicone-based ones prepared by emulsifying a polydimethylsiloxane in water by using a surface active agent belong to the class of the most widely used defoaming compositions.
Silicone-based defoaming compositions in the form of an aqueous emulsion, however, have a disadvantage relative to the stability. When the aqueous foaming liquid, to which the silicone-based defoaming composition is added, is heated at a high temperature or receives a high shearing force, for example, the emulsion of silicone is sometimes destroyed to cause a decrease in the defoaming activity or, in some cases, rather to promote foaming of the aqueous foaming liquid. Further, the storage stability of silicone-based defoaming compositions in the form of an aqueous emulsion in general is poor so that the phenomenon of phase separation is more or less unavoidable in a silicone-based defoaming composition of the aqueous emulsion-type after storage for a long period of time to isolate the polydimethylsiloxane forming a free oily phase. Once the phenomenon of phase separation takes place, the isolated polydimethylsiloxane exhibits water repellency so that a great decrease is caused in the quality of the printed matter prepared by using the water-based printing ink composition admixed with the defoaming agent.
In this connection, a proposal is made in U.S. Pat. No. 3,233,986 for the use of a polydimethylsiloxane as a principal ingredient of a defoaming composition, of which some of the methyl groups are replaced with polyoxyalkylene groups, but sufficiently high defoaming activity can hardly be obtained with a defoaming composition prepared by emulsifying such a polyoxyalkylene-modified polydimethylsiloxane. As a remedy for this low defoaming activity of the defoaming composition prepared from a polyoxyalkylene-modified polydimethylsiloxane, Japanese Patent Kokai No. 53-34854 proposes combined use of such a modified polydimethylsiloxane and an unmodified polydiorganosiloxane, e.g., polydimethylsiloxane, in order to enhance the defoaming activity of the defoaming composition. Such a defoaming composition added to a water-based printing ink composition, however, is defective due to eventual phase separation of the organopolysiloxane ingredients to exhibit repellency because the polydimethylsiloxane has relatively low affinity with the organic materials as the ingredients of the water-based printing ink compositions.
Attempts have of course been made to obtain a silicone-based defoaming composition particularly suitable for use in a water-based printing ink composition by introducing certain hydrocarbon groups different from methyl groups into a polydimethylsiloxane. For example, Japanese Patent Kokai No. 54-149388 proposes a self-emulsifiable silicone-based defoaming composition by the combined use of such a diorganopolysiloxane modified by introducing hydrocarbon groups having 4 to 18 carbon atoms and a polyoxyalkylene-modified polydiorganosiloxane. Though somewhat improved in respect of undesirable appearance of repellency, such a defoaming composition is unsatisfactory due to the poor sustainability of the defoaming activity.
With an object to provide a defoaming composition particularly suitable as an additive to a water-based printing ink composition with improved sustainability of the defoaming activity, the inventors in Japanese Patent Publication No. 1-30872 have previously proposed a silicone-based defoaming composition comprising a polydiorganosiloxane modified by introducing polyoxyalkylene groups of a specified structure and other modifying groups such as alkyl groups of 3 to 30 carbon atoms in combination with a polydimethylsiloxane, a finely divided silica powder and a surface active agent. The improvement achieved with this defoaming composition, however, is still not quite satisfactory in respect of the insufficient improvement in the sustainability of the defoaming activity when it is added to a water-based printing ink composition.